Transportable silos for dry granules and powders that are filled by pneumatic tank trucks after installation have been known for many years. They consist of a thin metal silo which is designed to hold the bulk powder or granular product. A fixed tanker unloading pipe extends from near ground level to the silo top. A flexible hose is connected between the pneumatic tank truck and the tanker unloading pipe and the tank is then pressurised to pneumatically convey the contents of the truck in the silo. Pneumatic tank trucks are designed to operate at either 1 bar(g) or 2 bar(g).
The air used for transporting the bulk material into the silo is in most cases vented to atmosphere through a self cleaning filter unit mounted on the silo roof. In some cases this filter unit is mounted at ground level and a duct connects the filter to the top of the silo. During unloading, there is a small pressure drop as the conveying air is pushed through the filter and for this reason the silo is designed to take a small positive pressure of typically 0.02 to 0.05 bar(g).
To enable the truck operator to know when the silo is full, a level probe is fitted at the highest acceptable material level in the silo. The silo may also be mounted on load cells so that the amount of bulk product in the silo is known. An alternative to the load cells is to fit a number of level probes at different levels in the silo so that the user can see when the silo is ½ full, ¼ full for example.
To enable the user to install and service the equipment such as the filter unit and level probes, an access ladder is provided from ground level to the silo top and the silo will have handrails and a standing area on the silo top for the safety of maintenance personnel.
The conveying air filtration unit has to be sized to take the flow of air during unloading of the product and also a very high airflow which comes from the tanker truck as the tanker truck becomes empty and the compressed air in the tanker truck vents down the now empty conveying pipe into the silo. Typical cloth filter areas required to cope with this are between 20m2 and 30m2. With this cloth area, the filter units are quite large.
Even though the silo may be a temporary one, it has to be installed on prepared concrete foundations that can take the static loads and overturning wind loads. This results in high cost even for a temporary silo installation.
Small temporary silos of this type will typically hold 20 to 50 tonnes of product. The silo is simply a holding device. If the material in the silo has to be moved some distance to the point of use, mechanical or pneumatic conveying systems must be installed under the silo outlet to move this material.
Because of the need for prepared foundations and the construction of onward conveying systems, the ‘temporary’ silo arrangement can be expensive to install and takes at least 3 to 4 days to complete after foundations have been completed. This adds to the total project cost. Planning permission may also be required for this fixed installation which could take many months to get.
Transportable silos are normally constructed with 4 support legs with cross bracing. They may be moved to site on one or more lorries and assembled at site. Whenever they are lifted, a crane or cranes are required with lifting chains or slings. Ladder and filter units are easily damaged and these silos cannot be moved on intermodal transport such as that used for containers or lifted with container handling devices.
The vessel of the present invention overcomes many of the problems associated with temporary or portable silos. Reference will be made to a type of silo which is mounted in, or capable of being mounted in, an ISO frame, such a vessel is referred to herein as an ISO-Silo™.